Investigation or diagnosis using X-rays is the nucleus of current medical image diagnosis. So-called hard tissues such as bones and teeth efficiently absorb X-rays and thereby high contrast X-ray images can be obtained. On the contrary, the difference in X-ray absorption between different soft tissues is relatively small, making it difficult to obtain high contrast images. In such cases, contrast mediums are generally used to obtain high contrast images.
Almost all X-ray contrast mediums which are currently of practical use are contrast medium materials which are water-solubilized compounds containing a triiodophenyl group. The contrast medium is given to a lumen region such as a vascular tract, a ureter or a uterine tube to be used for examination of a form or stenosis of the lumen. However, the foregoing compounds are promptly discharged from the lumen region without interacting with tissue or disease regions, which is not useful for detailed examination of the tissue or disease region, specifically such as cancer tissue. Therefore, an X-ray contrast medium has been desired which can be selectively accumulated in/or onto the targeted tissue or disease region, thereby giving an image which can be distinguished with clear contrast from the circumference or other regions.
A technique of transporting a contrast medium which has been fine-grained and improved in half-life in blood to the targeted tissue is effective to overcome the foregoing problems. There was studied a method in which a contrast medium compound was allowed to be included in a liposome which was comprised of a lipid similar to a biomembrane, and which exhibited low antigenicity. For example, International Publication WO88/09165, ibid WO89/00988, ibid WO90/07491; JP-A No. 07-316079 and 2003-5596 (hereinafter, the term JP-A refers to Japanese Patent Application Publication) propose a liposome containing a non-ionic contrast medium. Further, in the above-mentioned methods, although a liposome exhibiting high safety as raw material and optimal degradability in vivo, organic solvents, specifically chlorinated solvents such as chloroform and dichloromethane were used in the preparation process, as a solvent for phospholipid forming a liposome membrane. Accordingly, the foregoing methods were not practically applicable due to toxicity of retained solvents.
On the other hand, although chemicals soluble in lipid are easily included in a liposome, the included quantity, depending on other factors, is not necessarily large. Although water-soluble electrolytic chemicals can be included in a liposome through interaction of a charge of the chemicals with that of a charged lipid, such a means is not applicable to water-soluble non-electrolytic chemicals. It has been generally desired to allow non-ionic iodine compounds substantially exhibiting low toxicity to be included in a liposome rather than ionic contrast medium compounds, which is not easy from the foregoing reasons. Further, the-formed liposome easily formed a multi-layered membrane and the enclosure ratio of the iodine compound was low. Means for allowing a water-soluble non-electrolyte to be efficiently included in a liposome include, for example, a reversed phase evaporation method and an ether injection method. In these means, however, organic solvents are used, producing problems of safety.
JP-A No. 2003-119120 discloses a method of preparing liposome-containing cosmetics or skin medicines for external use by using supercritical carbon dioxide, which is exemplified in the preparation of a skin medicine for external use occluding hydrophilic or hydrophobic medicinal components in a liposome. However, although examples of a water-soluble electrolyte are shown therein as a medicinal component, it is unclear whether a water-soluble non-electrolyte is efficiently included in a liposome using this method. Further, this method is desired to use auxiliary solvents such as ethanol to enhance the enclosure ratio, rendering it difficult to prepare a liposome of a relatively high enclosure ratio without the use of organic solvents. Even if inclusion of a contrast medium material is done well, problems such as its leaking-out over an elapse of time or the situation of the liposome itself becoming unstable must be taken into account. It is further pointed out that since a liposome introduced into an organism is almost always trapped in a reticuloendothelial system such as the liver or spleen, the intended effects cannot be achieved, as described in Cancer Res., 43, 5328 (1983). When iodine compounds are included in a liposome, as is distinct from conventional radiographic contrast medium comprised of an iodine compound in a free form, the form of contrast medium material existing inside and outside of the liposome, the inclusion ratio thereof and the like affect performance of the contrast medium. Accordingly, there is still desired a method of preparing a liposome in which non-ionic iodine compounds are efficiently included and stably maintained, producing no problem in safety.